Release of dopamine from mesocorticolimbic synapses underlies important aspects of drug dependence, working memory, the psychosis associated with schizophrenia, and classical conditioning. This research is intended to elucidate the presynaptic mechanisms utilized by midbrain dopamine neurons and to provide a basis for how these mechanisms contribute to psychostimulant-induced alterations in neuronal function. The most significant presynaptic property, quantal release of neurotransmitter via synaptic vesicle exocytosis, has not previously been observable in catecholamine neurons as in fast- acting synaptic systems, due to lack of rapid postsynaptic currents. Recently, we introduced an approach to directly observe quantal release in central monoamine neurons. We found that amphetamine perturbs physiologically-appropriate dopamine release by reducing the number of molecules released per synaptic vesicle exocytic event (quantal size). Our continuing efforts show both quantal size and frequency is strongly regulated by D2-like dopamine autoreceptors, which would be expected to be maximally stimulated during psychostimulant exposure. The objective of this proposal is to determine basic mechanisms by which dopamine autoreceptors regulate dopamine release over durations of minutes to hours and to place this in a context that may elucidate how psychostimulant modulation of presynaptic function (both quantal release and stimulation- independent mechanisms) underlies effects of psychostimulants, such as amphetamine-mediated sensitization and disturbance of physiological DA input. A recent study by our laboratory has shown that in a dopamine (PC 12) cell line, quantal size is regulated by D2-like dopamine autoreceptor-mediated effects on tyrosine hydroxylase. We now present Preliminary Results suggesting that an analogous response occurs in presynaptic terminals of the midbrain dopaminergic projection neurons. Some alternate mechanisms are also addressed. We explain why regulation of quantal size would be an important plastic parameter, particularly for monoamine and other systems where extrasynaptic overt-ow occurs, and suggest how these non- acute, long-lasting autoreceptor-mediated effects in presynaptic dopamine terminals may provide important steps for the initiation of responses such as psychostimulant dependence and sensitization.